There's a principle (will try to find the source later) that says that if one is in the midst of doing one mitzvah he is exempt from performing other mitzvot.

Do both mitzvot have to be of the same type (biblical or rabbinical)? Or does any mitzvah count towards this principle.

A common application of this happens daily: You have 5 minutes until sof zman kriat shema (a biblical obligation) and you're in the middle of reciting pesukey dizimra (a rabbinical mitzvah). Should you interrupt pesukey dizimra to say shema on time? Or does reciting the pesukey dizimra exempt you from saying shema on time?

The question title and your contents cause some vagueness, and, as such, I had an answer which I temp. deleted. Your title specifies that you are in the midst of doing a Rabbinical mitzvah & you want to know if the opp. to do a Biblical one would cause you to stop doing the Rabbinical one. The question contents appear o be far more general than this. Please clarify what you seek.
– DanFMay 21 '15 at 19:57